SELECT *
FROM forum_posts AS pa
LEFT JOIN forum_posts_text AS pb ON pa.post_id = pb.post_id
LEFT JOIN forum_users ON user_id = post_poster
WHERE post_topic_id = ‘450′
ORDER BY pa.post_id ASC
LIMIT 5475 , 15

versus

SELECT *
FROM forum_posts AS pa
LEFT JOIN forum_posts_text AS pb ON pa.post_id = pb.post_id
LEFT JOIN forum_users ON user_id = post_poster
WHERE post_topic_id = ‘224′
AND pa.post_id >= (
SELECT MAX( post_id )
FROM (
SELECT post_id
FROM forum_posts
WHERE post_topic_id = ‘224′
ORDER BY post_id ASC
LIMIT 6075 , 1
) AS tmp )
ORDER BY pa.post_id ASC
LIMIT 15

~ 0.09 sec versus ~ 0.004 sec

Tables with posts: 2×292,000+ rows (25MB + 89MB)
Topic: 6092 rows (rows that meet condition WHERE post_topic_id = ‘224′)

###############
Later tests:

.. LIMIT 66240 , 15

versus

..
LIMIT 66240 , 1
..
LIMIT 15

~ 0.5 sec versus ~ 0.03 sec

Tables with posts: 2×377,000+ rows (33MB + 120MB)
Topic posts: 66259 rows (rows that meet condition WHERE post_topic_id = actual id)

I tested described optimization methods on my large database(near 1,2GB data) and here is the result:

mysql> SELECT id FROM ltable WHERE id>(SELECT id FROM ltable LIMIT 120000,1) limit 5;
+——–+
| id |
+——–+
| 581099 |
| 581100 |
| 581101 |
| 581102 |
| 581103 |
+——–+
5 rows in set (38.70 sec)

mysql> SELECT id FROM ltable LIMIT 120001,5;
+——–+
| id |
+——–+
| 581099 |
| 581100 |
| 581101 |
| 581102 |
| 581103 |
+——–+
5 rows in set (20.31 sec)

we can see that the method don’t work correctly… What do u think ?

for( $i=0; $i<strlen ($str)/2; $i++ )

That doesn’t make any sense :/

How to fix it ?

None